Answer
$$ - 3$$
Work Step by Step
$$\eqalign{
& 3\operatorname{cis} {180^ \circ } \cr
& = 3\left( {\cos {{180}^ \circ } + i\sin {{180}^ \circ }} \right) \cr
& {\text{Calculate }}\cos {180^ \circ }{\text{ and }}\sin {180^ \circ } \cr
& = 3\left( { - 1 + i\left( 0 \right)} \right) \cr
& = - 3 \cr} $$
